Plastic materials such as polymeric films have been widely used for packaging various food and non-food products. In order to insure proper preservation of products packaged in such polymeric films, it is necessary to provide the films with barriers against transmission of air, moisture, deleterious flavors, etc. No single unmodified polymeric film, however, has sufficient gas and moisture barrier characteristics needed for proper packaging requirements. For example, polyolefin films such as polypropylene films are particularly preferred in the manufacture of packaging films due to their low cost and ease of manufacture. Such polypropylene films, however, inherently permit the transmission of oxygen and water vapor from the outside of the film to the inside of the package made up of the film. When such films are used as food packagings, oxygen and water vapor transmitted therethrough promote rapid deterioration of foods packaged therein.
In order to provide acceptable barrier properties, multi layer polymeric films have been developed having improved water vapor transmission rates (WVTR). For example, incorporating wax into film structures has been known to improve water vapor transmission rates. It is believed that the wax migrates or blooms to the outside surface of the film structure and becomes crystalline, thus imparting superior WVTR and improved oxygen barrier properties. In this approach, however, the wax on the surface is susceptible to removal, thereby presenting difficulty in maintaining such WTVR and oxygen barrier properties, particularly when such films are used in laminating, printing and coating operations.
In order to provide a film having improved barrier properties, U.S. Pat. Nos. 5,141,801 and 5,155,160 disclose incorporating a wax into a polyolefin surface layer of a co-extruded film. To prevent migration of the wax to a surface which is to be used in lamination and printing, these patents suggest incorporating a barrier layer of polymeric material such as a polyamide or an ethylene vinyl alcohol copolymer (EVOH). Such polyamide and EVOH barrier layers, however, are expensive and difficult to manufacture. Thus, while the prior art discloses various useful films, such films are typically susceptible to removal of the wax during subsequent laminating, printing and coating processes, and are difficult and expensive to manufacture.
Accordingly, a need exists for a packaging film which is simple and inexpensive to manufacture, which is capable of providing enhanced barrier properties to oxygen and water vapor transmission, and which is capable of subsequent laminating, printing or coating procedures.